There are many applications where a response to information displayed on a CRT screen is desired. For example, a video game, which is played on a personal computer or a television set, is oftentimes programmed to display a menu of choices from which a user may make a selection. The user is typically instructed to operate specific keys on a keyboard or a similar device to accomplish this selection. Multiple menus are also often used where, in response to the user operating a selected key to make a selection, the menu changes and the user may make another selection, again making a selection by operating specific keys. The process of menu display and selection can be repeated many times. Such an arrangement is time consuming and tedious since a user must first look at the screen, determine a choice, and then examine a separate device to find the proper key to effectuate this choice.
Several attempts have been made to solve the above-identified problem, one such attempt being the use of a touch-sensitive screen. With such a screen, a particular choice can be selected from a menu by a user touching the area of the screen where that choice is displayed. Various techniques have been employed in the prior art to provide the touch-sensitive-screen capability. The use of perpendicularly disposed light beams is one such example. In this technique, infrared light beams are emitted from each of two perpendicular sides of a screen and are received by sensors. As such, the light beams travel across the screen horizontally and vertically. Each of the light beams is received by one of the sensors. When a finger touches a particular spot on the screen, a horizontal beam and a vertical beam are thus momentarily prevented from being received by their corresponding sensors. The touch-position is determined based on a knowledge of which two sensors are deprived of their otherwise receivable beams. This use of perpendicular light beams, while widely used, requires many special, physical and electrical arrangements to provide the touch-sensitive-screen capability and it is expensive to implement.
Many other touch-sensitive-screen techniques make use of special surfaces for the screen. One such technique requires that a screen be covered with two parallel resistive membranes. This particular technique relies on the fact that when a person touches the screen, the two membranes contact each other, and this causes the electrical resistance of the membranes to change. A circuit behind the screen determines the touch-position based on the variation in the resistance. However, the added membrane surfaces, required by this technique and its variants, not only incur additional costs but also undesirably degrade the resolution of the display. More importantly, all of the aforementioned prior art techniques, in one way or another, either call for specially designed monitors or require modifications to standard monitors or television sets.
There is, however, a touch-screen technique which can be applied to any monitor having a CRT, such as a television set. As is well known, images are created on a CRT screen as it is scanned by an electron-beam. This electron beam is generated inside the CRT and the scan is orchestrated by control circuitry inside the monitor. In accordance with such a touch-screen technique, a light pen is held against the image on the CRT screen corresponding to a desired selection. Light from that image then enters the pen wherein a photo-diode generates a signal output. The touch-position is determined based on the knowledge of the position of the electron-beam with respect to the screen at the time of the signal output. The use of a light pen also has shortcomings. One shortcoming is that light pens are susceptible to interference caused by ambient light. Another is that the light pen's photo-diode may not be durable enough for certain applications.
Accordingly, it is desirable to have a touch-screen device which can be used with any monitor having a CRT screen and which, unlike a light pen, is durable and unsusceptible to interference due to ambient conditions.